[SOLVED] gcc macro expansion of #error

Issue

I have some preprocessor code like this:

#define STR_HELPER(x) #x
#define STR(x) STR_HELPER(x)

#if GCC_VERSION < 40503
#error Your compiler is outdated.
#error You need at least gcc 4.5.3 for this library.
#error You have: GCC_VERSION    
#error You have: STR(GCC_VERSION)
#endif

However both ways I tried to output the current compiler version fail. According to the documentation this is because

Neither ‘#error’ nor ‘#warning’ macro-expands its argument. Internal whitespace sequences are each replaced with a single space. The line must consist of complete tokens. It is wisest to make the argument of these directives be a single string constant; this avoids problems with apostrophes and the like.

How can I output the compiler version with the error message anyway? Is there any preprocessor magic that allows me to achieve this?

Solution

Combining the proposal of paxdiablo and ideas for static compile time assertions I settled for the following solution.

#define GCC_VERSION (__GNUC__ * 10000 \
+ __GNUC_MINOR__ * 100 \
+ __GNUC_PATCHLEVEL__)

#define ERROR_MESSAGE(major, minor, patchlevel) compiler_version__GCC_ ## major ## _ ## minor ## _ ## patchlevel ## __ ;
#define OUTDATED_COMPILER_ERROR(major, minor, patchlevel) ERROR_MESSAGE(major, minor, patchlevel)

#if GCC_VERSION < 40503
#error Outdated compiler version < 4.5.3
#error Absolute minimum recommended version is avr-gcc 4.5.3.
#error Use 'avr-gcc --version' from the command line to verify your compiler version.
#error Arduino 1.0.0 - 1.0.6 ship with outdated compilers.
#error Arduino 1.5.8 (avr-gcc 4.8.1) and above are recommended.

OUTDATED_COMPILER_ERROR(__GNUC__, __GNUC_MINOR__, __GNUC_PATCHLEVEL__)
#endif

Answered By - Udo Klein

Answer Checked By - Gilberto Lyons (WPSolving Admin)